The first inductor in the SMPS

[abdan]

What or how exactly is the first inductor's function and working mechanism lying in most SMPS as shown ?

 

[Martaine2005]

It is a common mode choke. It cancels noise on the lines.
It is a suppressor. A filter if you like.

Martin

 

[Harald Kapp]

A guide to understanding common mode chokes.